Abbreviation for "overly attached girlfriend."
After adding an old friend on facebook, I got a message from his OAG: "Just wondering how you know him and why you're friends with him all of a sudden on Facebook when I've never heard of you before?"
"oag" is an austrian or rather viennese term that is used to acknowledge something while indicating that you are surprised/disturbed/curious about it.
it is a slang version of the german term "arg" but germans usually don't use it for the purpose mentioned above and illustrated in the example below.
Dude: My tutor gave me a kiss on my cheek in today's seminar!